HVAC systems are used to maintain a desired temperature in enclosed spaces that are subject to heat inputs and heat loss. Heat inputs may be internally produced within an enclosed space as a result of human occupancy and activity, and the operation of heat-generating apparatus such as computers and lighting systems (hereinafter “people and heat-generating equipment”). Heat inputs may also be externally applied, from solar radiation and convective heat transfer from outside air. On the other hand, heat may be lost from one enclosed space to another, and through exterior walls and windows.
Often, buildings in which HVAC systems are used have multiple levels (commonly referred to as “storeys” or “floors”). In general, each level will have perimeter walls and windows, and a floor and ceiling separating it from the levels immediately above and below. It will simplify the discussion to focus on just one level, it being understood that other levels may be treated the same; and hereinafter, the word “floor” will be used to refer to a level.
The HVAC system typically strives to maintain a constant temperature TDESIRED, typically about 70 degrees Fahrenheit, on a floor, year-round. To do this, the floor is typically divided into multiple “zones” of temperature control. There are two basic types of zones, namely, “interior” and “perimeter.” Interior zones are spaces enclosed on all sides by one or more of the perimeter zones. The significance of this relationship is that, if the temperature is controlled to be the same in the interior and perimeter zones—which it typically will be—there can be no heat loss from an interior zone. Rather, interior zones will always experience heat gain so long as there are any heating inputs, meaning so long as there are either people or heat-generating equipment within the interior zones. This establishes a need, year-round, to provide a “cold air supply” to the floor at a temperature TLOWER, typically, about 55 degrees F.
In the basic prior art HVAC system, the building is provided with a cold air supply duct, that runs from an “air handler” portion of the HVAC system to the floor. The air handler must be exposed to outside air. It often resides on the roof of the building, or in a vented mechanical room within the building, or it may be installed outside the building. The air handler supplies the cold air supply duct with air at the lower temperature TLOWER needed to cool all the interior zones. In turn, the cold air supply duct supplies cold air to both the perimeter and interior zones.
The building is also provided with a “return air” duct that ducts air from the floor to the air handler. So the return air duct completes a circuit, which is easiest to visualize in the case of interior zones, wherein relatively cold air (at the temperature TLOWER) is supplied from the air handler to the cold air supply duct, from the cold air supply duct to the zones, warmed by people and equipment in the zones (to the temperature TDESIRED), and returned to the air handler through the return air duct (at the temperature TDESIRED).
The air is circulated by fans at the air handler. The rotational velocity of the fans are varied to match the airflow.
Within each zone, there are one or more “air supply units” that connect to the cold air supply duct, and controllably vary the amount of airflow from the cold air supply duct into the zone. Each air supply unit also includes means for heating the air received from the cold air supply duct, typically an electric or hot water coil (hereinafter, the term “heating coil” will refer generally to these and any other standard means of heating air). The air supply units vary the volume of the airflow and/or heat the air according to a need established by a temperature sensor and a feedback control system using TDESIRED as the set-point. Thus, while each zone may have any number of air supply units, the air-flow and heating requirements at all of the air supply units of the same zone will be the same, controlled by reference to the output of a single temperature sensor associated with that zone.
To supply the cold air supply duct, the air handler is capable of mixing outside air with return air in any proportion desired, and further cooling the air, if needed, by means of a refrigerating coil, or a coil carrying either refrigerated or unrefrigerated water (hereinafter, the term “cooling coil” will refer generally to these and any other standard means of cooling air), to achieve the temperature TLOWER.
When the outside air temperature is HOT (substantially higher than the temperature TDESIRED), the air handler will draw the minimum volume of outside air necessary for ventilation purposes, so a large percentage of the return air (at the temperature TDESIRED) is recycled through the building.
Assuming that all the zones have substantially equal heating inputs from people and heat-generating equipment, the most cooling will be required in a perimeter zone because, in addition to the heat gain from people and heat-generating equipment as in interior zones, there is heat gain through the walls of the building in the perimeter zones under HOT conditions.
All the zones (perimeter and interior) will require cooled air under HOT conditions. But there will generally be one zone that requires the most cooling, establishing the cold air supply temperature TLOWER, assuming maximum airflow into that zone.
At other zones where less cooling is needed, less cooling is provided by diminishing the flow of cold air through the air supply units at those zones.
On the other hand, when the outside air temperature is COLD (substantially lower than the temperature TDESIRED), no cooling is needed to produce the cold air supply—it is sufficient for the air handler to mix cold outside air with return air to produce the cold air supply (at the temperature TLOWER). There will be heat loss in the perimeter zones, so the air supply units in the perimeter zones must heat the cold air supply by use of the heating coil.
Between the extremes of HOT and COLD outside air temperatures, there are transitional temperature circumstances which can be defined according to whether (A) for all the perimeter zones, the heating inputs (from people and heat-generating equipment in the zone, and the influence of outside air temperature and solar radiation impacting external walls and transmitted through windows) exceeds the heat loss to the external environment (through external walls and windows), in which case no heating is required in any of the zones (hereinafter “temperature circumstance (A)), or (B) there is at least one perimeter zone in which the heat loss exceeds the heat inputs, so that heating will be required at the air supply unit of any such perimeter zone, requiring use of the associated heating coil (hereinafter “temperature circumstance (B)).
As a further refinement, it was just noted previously that under HOT conditions, all the zones will require cooled air, though some will require less cooling than others; and at the zones where less cooling is needed, less cooling is provided by diminishing the flow of cold air through the air supply units at those zones. However, there is a minimum level of airflow that must be provided at each air supply unit to provide adequate ventilation for the zone. It may be that, even at this minimum level of airflow, too much cooling would occur in a particular zone. Since the airflow cannot be diminished any further, the air supply unit at that zone must apply the heating coil to heat the air (hereinafter “temperature circumstance (C)).
So, under temperature circumstances (B) or (C) the prior art HVAC system heats at least some of the air drawn from the cold air supply by use of one or more heating coils at air supply units serving the perimeter zones.
In a refinement of the basic system, a “hot air supply duct” is provided in the building, so there are now three ducts: (1) a cold air supply duct; (2) a hot air supply duct; and (3) a return air duct. And, in addition to a cooling coil for cooling outside/return air as needed to provide the cold air supply, the air handler has a heating coil for heating outside/return air as needed to provide the hot air supply.
The air supply units are adapted to draw from either the cold air supply duct, in zones requiring cooling, or the hot air supply duct, in zones requiring heating, and to match the cooling or heating needs by varying the level of airflow. However, again, there is a minimum airflow requirement at each of the air supply units, to provide for adequate ventilation. If some cooling is required but too much cooling would be produced at a particular zone at the minimum level of flow of air from the cold air supply, the air supply unit mixes air from the hot and cold air supplies to produce the required heating. Essentially, the heating that would have been provided by use of a heating coil at the air supply unit is provided, instead, by use of a heating coil in the air handler.
It is an object of the present invention to provide for more efficient HVAC systems.